Asphalt is the principal material used for constructing roads. A standard mix of asphalt generally comprises approximately 94% grade aggregate and 6% asphalt cement. The standard mix is prone to rutting and cracking. While the standard mix is initially low in cost, it has a high maintenance cost and it contains no recycled materials.
Many attempts have been made to improve the wear characteristics of the roads. Materials such as polyethylene, glass, sand and drywall have been added to the asphalt to improve its characteristics. The use of these materials and their ineffectiveness is widely known and documented.
Previous attempts have failed to differentiate between the problems associated with rutting, ductility, tensile strength, elasticity and adhesion, referred to as the polymer related properties and the problems associated with post compaction rutting and durability, referred to as aggregate related.
Styrene Butadiene Rubber latex (SBR) has been added to asphalt for road construction. A polymer modified mix generally comprises approximately 94% graded aggregate, 5.76% asphalt cement and 0.24% polymer, either SBR or an equivalent polymer such as the polymers available under the trademarks KRATON SBS, STYRELF SBS, GOODRICH and BASF SBR and DUPONT EVA.
The addition of SBR to the asphalt results in improved crack resistance and heat rutting resistance over the standard mix of asphalt. However, the addition of SBR does not have a significant effect on the polymer modified asphalt's ability to resist post compaction rutting. Further, although the polymer modified asphalt is low in maintenance, it is expensive to manufacture and does not contain any recycled materials.
Attempts have been made to add rubber to the standard asphalt mix. A rubber modified mix generally comprises approximately 90% grade aggregate, 7% asphalt cement and 3% rubber.
The rubber modified asphalt contains recycled material and has good post compaction characteristics. However in practise, the rubber modified asphalt fails frequently, has a high rate of maintenance since it exhibits poor aggregate adhesion, poor heat rutting and poor crack resistance characteristics.
The addition of rubber to asphalt hash not been satisfactory since there are three major disadvantages in using rubber in asphalt. First, fine rubber melts during processing diminishing its desired aggregate enhancing properties. Second, porous rubber absorbs the asphalt cement thus reducing the asphalt's ability to bond the aggregate. Third, if the grade of rubber is too large processing of the asphalt can become difficult.
Traditionally, asphalt is manufactured by heating and mixing the components together to form a slurry which can be deposited onto a road surface for compaction. However, the mere addition of rubber in a polymer modified asphalt (as illustrated in EP-A-0 049 485 and FR-A-2 580 658) using the traditional method does not produce suitable results.